Ion-exchange membranes play an important role in separation and purification processes. These membranes generally consist of either highly swollen gels or microporous structures with fixed charges derived from ionizable functional groups localized at the pore walls (Strathmann, H., In Synthetic Membranes: Science, Engineering and Applications; Bungay, P. M.; Lonsdale, H. K.; de Pinho, M. N., Eds.; NATO ASI Series C: Mathematical and Physical Sciences Vol. 181; D. Reidel Publishing Company: Dordrecht, Holland, (1986), pp 1-37). A membrane that contains fixed positive charges is called an anion-exchange membrane, and a membrane bearing fixed negative charges is called a cation-exchange membrane. The general purpose of ion-exchange membranes is not to exchange ions but to transmit them in a controlled way (Meares, P., In Mass Transfer and Kinetics of Ion Exchange; Liberti, L.; Helffefich, F. G., Eds.; NATO ASI Series E: Applied Science No. 71; Martinus Nijhoff Publishers, The Hague, The Netherlands, (1983); pp 329-366). Co-ions (i.e., ions with the same charges as the fixed charges) are excluded from the pores, whereas counterions (i.e., ions with opposite charges to the fixed charges) selectively transport across the membrane. Ion-exchange membranes have been used in the following processes, classified by the driving forces of transport of ions (Meares, P., In Mass Transfer and Kinetics of Ion Exchange; Liberti, L.; Helffefich, F. G., Eds.; NATO ASI Series E: Applied Science No. 71; Martinus Nijhoff Publishers, The Hague, The Netherlands, (1983); pp 329-366): (a) for electrical driving forces, desalination, demineralization, concentration of solutions, exchange of ions, and oxidation-reduction (e.g., chlor-alkali processes); (b) for driving forces of concentration gradient, diffusion dialysis, solid electrolytes in batteries, and ion-selective electrodes, (c) for driving forces of pressure, reverse osmosis and piezodialysis.
The most common functional groups in cation-exchange membranes are sulfonic acid (SO3H) and carboxylic acid (xe2x80x94COOH) groups. The Nafion brand perfluorosulfonated polymer membranes (Meares, P., In Mass Transfer and Kinetics of Ion Exchange; Liberti, L.; Helffefich, F. G., Ed.; NATO ASI Series E: Applied Science No. 71; Martinus Nijhoff Publishers, The Hague, The Netherlands, (1983); pp 329-366; Yeager, H. L. et al., In Perfluorinated Ionomer Membranes; Yeager, H. L.; Eisenberg, Eds.; ACS Symposium Series 180; American Chemical Society: Washington, D.C., (1982); pp 1-6) are an example of the first type. These membranes were developed by E. I. du Pont de Nemours and CO. during the 1960""s and are still extensively used in industry. The perfluoro-sulfonic acid is ionized at normal pH values because of their low pKa values ( less than 1) (Seko, M. et al., ibid.; pp 365-410). In the seventies some Japanese companies (Asahi Chemical Co., Asahi Glass Co., and Tokuyama Soda Co.) developed perfluorocarboxylated membranes that contain carboxylic acid groups (Seko, M. et al., ibid.; pp 365-410; Sata, T. et al., ibid.; pp 411-425; Ukihastfi, H. et al., ibid.; pp 427-451; Sato, K. et al., PolymerJ., 23, 1991, 531-540). Although the perfluorocarboxylic acid has a higher pKa (2-3), it can be nearly completely ionized even in weakly acidic environments. Other functional groups such as phosphonic acid (xe2x80x94PO3H2) and sulfonamide (xe2x80x94SO2NH2) are sometimes used but are less practical.
The functional groups in anion-exchange membranes are usually quaternary ammonium [xe2x80x94N+(CH3)3] and to a lesser extent quaternary phosphonium [xe2x80x94P+(CH3)3] and tertiary sulfonium [xe2x80x94S+(CH3)2]. Anion-exchange membranes are frequently less stable than cation-exchange membranes because the basic groups are inherently less stable than the acidic groups (Strathmann, H. In Synthetic Membranes: Science, Engineering and Applications; Bungay, P. M.; Lonsdale, H. K.; de Pinho, M. N., Eds.; NATO ASI Series C: Mathematical and Physical Sciences Vol. 181; D. Reidel Publishing Company: Dordrecht, Holland, (1986); pp 1-37).
Conventional ion-exchange membranes are generally produced by: (a) polymerization (condensation or addition) of ionogenic monomers; (b) introduction of ionizable groups into a polymer film by grafting and/or chemical treatment; or (c) heterogeneously dispersing an ion exchange material into a binder polymer matrix.
The Nafion perfluorosulfonated membranes are copolymers of tetrafluoroethylene and perfluorovinyl ethers containing sulfonic acid groups (Yeager, H. L. et al., In Perfluorinated Ionomer Membranes; Yeager, H. L.; Eisenberg, Eds.; ACS Symposium Series 180; American Chemical Society: Washington, DC, (1982); pp 1-6). Synthesis of perfluorocarboxylated membranes is similar, but the sulfonic acid groups in perfluorovinyl ethers are replaced by carboxylic acid groups (Seko, M. et al., ibid.; pp 365-410; Sata, T. et al., ibid.; pp 411-425; Ukihastfi, H. et al., ibid.; pp 427451; Sato, K. et al., PolymerJ., 23, 1991, 531-540).
Perfluorinated ionomer (i.e., ion-containing polymer) membranes constitute a significant portion of ion-exchange membranes and are extensively used in the chlor-alkali industry (Dotson, R. et al., In Perfluorinated Ionomer Membranes; Yeager, H. L. Eisenberg, Eds.; ACS Symposium Series 180; American Chemical Society: Washington, D.C., 1982, pp 311-364). Composite membranes are sometimes utilized to improve the separation performance. An exemplary composite membrane has layers of both sulfonic acid and carboxylic acid polymers bound together to improve permselectivity (Sato, K. et al., PolymerJ., 23, 1991, 531-540). Composite layers having a layer of polyanion adsorbed onto a cation-exchange membrane have been prepared. Such membranes could prevent the precipitation of hydroxides and simplify the control of membrane fouling (Meares, P., In Mass Transfer and Kinetics of Ion Exchange; Liberti, L.; Helffefich, F. G., Eds,; NATO ASI Series E: Applied Science No. 71; Martinus Nijhoff Publishers, The Hague, The Netherlands, (1983); pp 329-366). Recent development in synthesis of ion-exchange membranes focus on new polymerization processes, such as radiation induced grafting (Chakravorty, B. et al., Membr. Sci. 1989, 41, 155-161; Gineste, J.-L. et al., Polym. Sci.: Part A: Polym. Chem. 1993, 31, 2969-2975) and copolymerization, (,G. K. et al., Membr. Sci. 1992, 68, 133-140) plasma polymerization of ionomer films (Brumlik, C. J. et al., Electrochem. Soc. 1994, 141, 2273-2279), deposition of thin polymer film by plasma polymerization (Osada, Y. In Membrane Science and Technology; Osada, Y.; Nakagawa, T., Eds.; Marcel Dekker, Inc.: New York, 1992; pp 167-201), and grafting of functional groups on polymers treated by ozonization Elmidaoui, A. et al., Appl. Polym. Sci. 1991, 42, 2551-2561).
In conventional ion-exchange membranes prepared by polymer chemistry, ion transport operates in a gel phase formed by sorption of water and swelling of the membrane due to the hydrophilic functional groups on the polymer backbone (Leddy, J. J. In Synthetic Membranes; Chenowetb, M. B., Ed.; MMI Press Symposium Series; Harwood Academic Publishers: London, 1986; pp 119-128). The size of pores, however, is difficult to control and there can be undesired transport of water and co-ions across the membrane, leading to poor perm-selectivity. Perfluorocarboxylated membranes are believed to have higher permselectivity than the Nafion membranes due to less uptake of water by the carboxylic groups than by the sulfonic groups (Sato, K. et al., PolymerJ., 23, 1991, 531-540).
Ion-exchange membranes with a porous structure were recently prepared by several techniques to offer membranes with both suitable pore sizes and good ion exchange capacity. These techniques include oxidative etching of gel-like ion-exchange membranes (Mizutani, Y. et al., J. Appl. Polym. Sci. 1990, 39, 1087-1100), chemical modification of preformed ultrafiltration membranes (Breitbach, L. et al., Angew. Makromol. Chem. 1991, 184, 183-196), and removal of inorganic fillers from a polymer blend and modification of the polymer matrix (Bryjak, M. et al., Angew. Makromol. Chem. 1992, 200, 93-108). Using those techniques, however, it is not flexible to introduce various functional groups and the pore sizes of the membranes are usually not uniform.
Martin et al. recently reported electroless deposition of gold onto the pore walls in polycarbonate track-etched (PCTE) filtration membranes (Menon, V. P. et al., Anal. Chem. 1995, 67, 1920-1928; Nishizawa, M. et al., Science 1995, 268, 700-702; Jirage, K. B. et al., Science 1997, 278, 655-658). Such gold-coated membranes have unique properties. Application of a positive or a negative electrical potential to gold resulted in anion or cation selectivity, respectively and the ion selectivity was reversibly altered by manipulation of the applied electrical potential (Nishizawa, M. et al., Science 1995, 268, 700-702).
Self-assembled monolayers formed with xcfx89-substituted alkanethiols on the surface of gold have been used as model surfaces in a number of past studies of the interactions of proteins with surfaces (Spinke et al., Langumuir, 9: 1821 (1993); Willner et al., J. Am. Chem. Soc., 114: 10965 (1992); Song et al., J. Phys, Chem., 97: 6564 (1993); Mrksich et al., J. Am. Chem. Soc., 117: 12009 (1995)). For example, multilayer systems based on biotinylated alkanethiols and streptavidin have been used in schemes for the immobilization of Fab fragments of antibodies on surfaces (Spinke et al., Langumuir, 9: 1821 (1993)), and SAMs formed from NHS-activated disulfidies have been used to form enzyme-based electrodes by covalent immobilization of glutathione reductase (Willner et al., J. Am. Chem. Soc., 114: 10965 (1992)). Cytochrome c, when adsorbed to SAMs formed from mercaptoundecanoic acid, has also been shown to be active and to possess a formal potential nearly identical to that of cytochrome c bound to physiological membranes (Song et al., J. Phys, Chem., 97: 6564 (1993)).
Whereas, investigations such as those described above have firmly established the use of SAMs for studies of specific interactions between proteins and surfaces, mixed SAMs formed from hydrophobic (methyl-terminated) and hydrophilic (hydroxyl-, oligo(ethylene glycol)-terminated) alkanethiols have also been used as model surfaces in studies of non-specific adsorption of proteins onto surfaces. Whitesides and coworkers, for example, have reported a study of the non-specific adsorption of fibrinogen, lysozyme, pyruvate kinase and RNAse to mixed SAMs (Prime et al., J. Am. Chem. Soc., 115: 10714 (1993); Prime et al., Science, 252: 1164 (1991)). By using ellipsometry, SAMs formed from oligo(ethylene glycol)-terminated alkanethiols were shown to resist irreversible adsorption of these proteins.
Surfaces prepared by the chemisorption of organosulfur compounds on evaporated films of gold are not limited to the alkanethiols. Self-assembled monolayers formed from perfluorinated organosulfur compounds have also been reported. See, Lenk et al., Langmuir, 10: 4610 (1994); Drawhorn et al., J. Phys. Chem., 99: 16511 (1995). These surfaces, too, can be highly ordered, although, interestingly, the origin of the order within the monolayer is largely intramolecular and contrasts, therefore, to monolayers formed from alkanethiols (where the order largely reflects the cohesive intermolecular dispersion force). Steric interactions between adjacent fluorine atoms of a perfluorinated chain cause the chain to twists itself into a rigid, helical conformation. That is, an isolated perfluoro chain is stiff, as compared to an aliphatic chain. Because perfluorinated chains have larger cross-sectional areas than alkanethiols, monolayers formed on gold from perfluorinated thiols are not tilted from the normal to the same degree as alkanethiols. See, Drawhorn et al., J. Phys. Chem., 99: 16511 (1995). Estimates by IR studies place the tilt of the perfluorinated chains at 0xcx9c10xc2x0. Because perfluorinated chains within SAMs on Au(111) are not tilted to the same degree as the alkanethiols, their surfaces are not expected to possess domains formed from regions of monolayer with different tilt directions (as occurs with monolayers formed from alkanethiols).
Gold-coated PCTE membranes were recently reported. See, Nishizawa, M. et al., Science 1995, 268, 700-702). These membranes were derivatized with 1-propanethiol to protect the gold from adventitious binding of anions (such as Clxe2x88x92, Brxe2x88x92, and Ixe2x88x92) presented in external solutions. These workers, however, did not investigate whether 1-propanethiol (or any alkanethiol) formed close-packed monolayers on the electroless gold coated membrane under the conditions used to prepare the membranes. Further, no suggestion was made to derivatize the 1-propanethiol layer with ionic or other groups to impart functionality and recognition properties to the membranes.
Easily prepared and characterized membranes that are capable of presenting a wide range of recognition groups (ionic groups, metal, complexing agents, biomolecules, and the like), pore sizes, surface charges and surface hydrophilicity/hydrophobicity would represent a significant advance in membrane science. Quite surprisingly, the present invention provides such membranes and methods of making and using these membranes.
It has now been discovered that membranes coated with metal films can be functionalized with SAMs (mixed and homogeneous) formed from organic groups bearing recognition moieties. Because a wide range of recognition moieties can be easily introduced onto surfaces by these methods, these membranes are useful for a range of purification methods and assays.
Thus, in a first aspect, the present invention provides a multilayered material comprising:
(a) a porous substrate;
(b) a metal film adhered onto said porous substrate;
(c) an organic layer attached to said metal film, said organic layer comprising a recognition moiety.
In a second aspect, the invention provides a multilayered material comprising:
(a) a polycarbonate track-etched substrate;
(b) a metal film adhered onto said substrate; and
(c) an organosulfur layer attached to said metal film, said organosulfur layer comprising a recognition moiety.
In a third aspect, the present invention provides an ion exchange medium comprising:
(a) a porous substrate;
(b) a metal film adhered onto said substrate; and
(c) an organic layer attached to said metal film, said organic layer comprising a recognition moiety that interacts with said ion.
In a fourth aspect, the present invention provides a method for removing an ion from a fluid, said method comprising:
(a) contacting said fluid with an ion exchange medium comprising:
(i) a porous substrate;
(ii) a metal film adhered onto said substrate; and
(iii) an organic layer attached to said metal film, said organic layer comprising a recognition moiety that interacts with said ion.
In a fifth aspect, the present invention provides a method for isolating a molecule from other molecules by affinity dialysis comprising:
(a) contacting the molecule with a multilayered porous material comprising;
a porous substrate;
a metal film adhered onto the substrate; and
an organic layer attached to the metal film, the organic layer comprising a recognition moiety.
(b) forming a complex between the recognition moiety and the molecule.
In a sixth aspect, the present invention provides a method of isolating a first molecule from a second molecule by size exclusion dialysis, comprising:
(a) contacting the first and second molecule with a multilayered porous material comprising;
a porous substrate;
a metal film layered onto the substrate;
a hydrophilic polymer attached to the metal film; and
passing the first molecule through the porous material while the second molecule is substantially retained thereby.
In a seventh aspect, the present invention provides a method for determining the presence or amount of an analyte in a test sample comprising:
(a) contacting the test sample with a multilayered porous material comprising;
a porous substrate;
a metal film layered onto the substrate;
an organic layer adhered to the metal film, the organic layer comprising a recognition moiety;
(b) forming a complex between the recognition moiety and the analyte; and
(c) detecting the analyte.
In an eighth aspect, the invention provides a method of producing a multilayered porous material comprising:
(a) contacting a porous substrate with a metal plating means to form a porous substrate having a metal film adhered thereto;
(b) contacting said porous substrate having a metal film adhered thereto with a plurality of organic molecules that associate with said metal film, wherein at least a portion of said plurality of organic molecules comprise a member selected from the group consisting of recognition moieties, reactive groups, protected reactive groups and combinations thereof.
In a ninth aspect, the present invention provides a drug delivery device comprising:
(a) a porous substrate;
(b) a metal film adhered onto said substrate;
(c) an organic layer attached to said metal film, said organic layer containing a recognition moiety; and
(d) a drug moiety reversibly associated with said recognition moiety.
Additional objects and advantages of the invention will be apparent to those of skill in the art from the detailed description and the examples that follow.